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Examining the effect of fertility on lifespan using data from the Swedish Twin Registry

EXAMINING THE EFFECT OF FERTILITY ON LIFESPAN USING DATA FROM
THE SWEDISH TWIN REGISTRY
by
Elizabeth Chereji
A Thesis Presented to the
FACULTY OF THE GRADUATE SCHOOL
UNIVERSITY OF SOUTHERN CALIFORNIA
In Partial Fulfillment of the
Requirements for the Degree
MASTER OF ARTS
(PSYCHOLOGY)
December 2009
Copyright 2009 Elizabeth Chereji

The disposable soma theory suggests that reproduction occurs at the expense of physiological resource maintenance (Kirkwood, 1977). Elaborating on this theory, some researchers have proposed that a negative relationship exists between longevity and reproduction (e.g. Westendorp & Kirkwood, 1998). Studies examining this relationship were often restricted to isolated groups that may be considered questionable representations of the general population (Westendorp & Kirkwood, 1998, Korpelainen, 2000). The current study examined whether the number of children a woman bears during her lifetime decreases her lifespan, as posited by the disposable soma theory. Female and male twin pairs from the Swedish Twin Registry were analyzed using a co-twin control model and survival analysis. Twin data helps elucidate whether this phenomenon is accounted for by familial factors or non-shared environmental factors. Based on the results of the survival analyses, there were main effects of birth year, gender, and having any children in predicting lifespan. More specifically, females, individuals who had children, and individuals born in later cohorts versus earlier cohorts had increased chances of survival. Also, birth year accounted for part of the association between having any children and lifespan. The results are consistent with a causal interpretation at the individual level, as the inclusion of co-twin information did not increase the predictability of survival. The evidence in this study does not support the disposable soma theory and sheds light on other factors that might affect lifespan.

EXAMINING THE EFFECT OF FERTILITY ON LIFESPAN USING DATA FROM
THE SWEDISH TWIN REGISTRY
by
Elizabeth Chereji
A Thesis Presented to the
FACULTY OF THE GRADUATE SCHOOL
UNIVERSITY OF SOUTHERN CALIFORNIA
In Partial Fulfillment of the
Requirements for the Degree
MASTER OF ARTS
(PSYCHOLOGY)
December 2009
Copyright 2009 Elizabeth Chereji